Energy is required for all vital functions of the body at the cellular and organ level and this need is met by the dietary intake of energy substrates. Energy produced during oxidation of nutrients is generally converted to ATP (adenosine-5-triphosphate) that, in turn, provides the energy for necessary activities when it is hydrolyzed to ADP (adenosine-5-diphosphate). In this chapter some general aspects of energy metabolism are summarized briefly. This review is followed by discussions on energy needs of term and preterm infants. Finally, protein-energy metabolism and balance with specific attention to protein-energy interaction are discussed.

The energy requirement for an individual has been defined as: the amount of energy intake from food that will balance energy expenditure and exogenous energy losses when the individual's body size and composition and physical activity profile is consistent with long-term good health. In children and pregnant or lactating women, the energy requirements also include the energy needs associated with deposition of tissues or secretion of milk consistent with good health. Requirements for energy during the neonatal period when referenced to body weight are higher than any time later in life, primarily because of high rates of growth.

Energy requirements can be best understood by examining the energy balance equation: Gross Energy intake = Energy excreted + Energy expended + Energy stored. The gross energy intake is the energy provided by the diet.